A strange lonely planet found without a star

An international team of astronomers has discovered an exotic young planet that is not orbiting a star. This free-floating planet, dubbed PSO J318.5-22, is just 80 light-years away from Earth and has a mass only six times that of Jupiter. The planet formed a mere 12 million years ago—a newborn in planet lifetimes.

It was identified from its faint and unique heat signature by the Pan-STARRS 1 (PS1) wide-field survey telescope on Haleakala, Maui. Follow-up observations using other telescopes in Hawai’i show that it has properties similar to those of gas-giant planets found orbiting around young stars. And yet PSO J318.5-22 is all by itself, without a host star.

“We have never before seen an object free-floating in space that that looks like this. It has all the characteristics of young planets found around other stars, but it is drifting out there all alone,” explained team leader Michael Liu of the Institute for Astronomy at the University of Hawai‘i at Mānoa. “I had often wondered if such solitary objects exist, and now we know they do.”

During the past decade, extrasolar planets have been discovered at an incredible pace, with about a thousand found by indirect methods such as wobbling or dimming of their host stars induced by the planet. However, only a handful of planets have been directly imaged, all of which are around young stars (less than 200 million years old). PSO J318.5-22 is one of the lowest-mass free-floating objects known, perhaps the very lowest. But its most unique aspect is its similar mass, color, and energy output to directly imaged planets.

“Planets found by direct imaging are incredibly hard to study, since they are right next to their much brighter host stars. PSO J318.5-22 is not orbiting a star so it will be much easier for us to study. It is going to provide a wonderful view into the inner workings of gas-giant planets like Jupiter shortly after their birth,” said Niall Deacon of the Max Planck Institute for Astronomy in Germany and a co-author of the study.

PSO J318.5-22 was discovered during a search for the failed stars known as brown dwarfs. Due to their relatively cool temperatures, brown dwarfs are very faint and have very red colors. To circumvent these difficulties, Liu and his colleagues have been mining the data from the PS1 telescope. PS1 is scanning the sky every night with a camera sensitive enough to detect the faint heat signatures of brown dwarfs. PSO J318.5-22 stood out as an oddball, redder than even the reddest known brown dwarfs.

Multicolor image from the Pan-STARRS1 telescope of the free-floating planet PSO J318.5-22, in the constellation of Capricornus. The planet is extremely cold and faint, about 100 billion times fainter in optical light than the planet Venus. Most of its energy is emitted at infrared wavelengths. The image is 125 arcseconds on a side. Credit: N. Metcalfe & Pan-STARRS 1 Science Consortium

“We often describe looking for rare celestial objects as akin to searching for a needle in a haystack. So we decided to search the biggest haystack that exists in astronomy, the dataset from PS1,” said Eugene Magnier of the Institute for Astronomy at the University of Hawaii at Manoa and a co-author of the study. Magnier leads the data processing team for PS1, which produces the equivalent of 60,000 iPhone photos every night. The total dataset to date is about 4,000 Terabytes, bigger than the sum of the digital version of all the movies ever made, all books ever published, and all the music albums ever released.

The team followed up the PS1 discovery with multiple telescopes on the summit of Mauna Kea on the island of Hawaii. Infrared spectra taken with the NASA Infrared Telescope Facility and the Gemini North Telescope showed that PSO J318.5-22 was not a brown dwarf, based on signatures in its infrared light that are best explained by it being young and low-mass.

By regularly monitoring the position of PSO J318.5-22 over two years with the Canada-France-Hawaii Telescope, the team directly measured its distance from Earth. Based on this distance, about 80 light-years, and its motion through space, the team concluded that PSO J318.5-22 belongs to a collection of young stars called the Beta Pictoris moving group that formed about 12 million years ago. In fact, the eponymous star of the group, Beta Pictoris, has a young gas-giant planet in orbit around it. PSO J318.5-22 is even lower in mass than the Beta Pictoris planet and probably formed in a different fashion.

The discovery paper of PSO J318.5-22 is being published by Astrophysical Journal Letters and is available at http://arxiv.org/abs/1310.0457. The other key authors of the paper are Katelyn Allers (Bucknell University), Trent Dupuy (Harvard-Smithsonian Center for Astrophysics), and Michael Kotson and Kimberly Aller (University of Hawai‘i at Mānoa).

Founded in 1967, the Institute for Astronomy at the University of Hawai‘i at Mānoa conducts research into galaxies, cosmology, stars, planets, and the sun. Its faculty and staff are also involved in astronomy education, deep space missions, and in the development and management of the observatories on Haleakala and Mauna Kea. The Institute operates facilities on the islands of Oahu, Maui, and Hawaii.

The Pan-STARRS1 Surveys (PS1) have been made possible through contributions of the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, Queen’s University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, the National Aeronautics and Space Administration under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation under Grant No. AST-1238877, the University of Maryland, and Eotvos Lorand University.

The Gemini Observatory is an international collaboration with two identical 8-meter telescopes. The Frederick C. Gillett Gemini Telescope is located on Mauna Kea, Hawai’i (Gemini North) and the other telescope on Cerro Pachón in central Chile (Gemini South); together the twin telescopes provide full coverage over both hemispheres of the sky. The telescopes incorporate technologies that allow large, relatively thin mirrors, under active control, to collect and focus both visible and infrared radiation from space.

The NASA Infrared Telescope Facility, a 3.0-meter infrared telescope dedicated to planetary science, is operated by the University of Hawaii under Cooperative Agreement no. NNX-08AE38A with the National Aeronautics and Space Administration, Science Mission Directorate, Planetary Astronomy Program.

Based on observations obtained with WIRCam, a joint project of Taiwan, Korea, Canada, France and the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institute National des Sciences de l’Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawai’i.